Funding for this research was provided by:
Capital's Funds for Health Improvement and Research (2018-1-1012)
Received: 29 November 2019
Accepted: 14 July 2020
First Online: 23 July 2020
Ethics approval and consent to participate
: This study was approved by the Ethics Committee of Beijing CDC. Written informed consent was obtained from all the participants or the guardians if participants were under 16 years of age.
: Not applicable.
: The authors declare no competing interests.